Method and apparatus for reselecting relay based on sl rlf

ABSTRACT

Provided are a method for performing wireless communication by a first device, and an apparatus for supporting same. The method may comprise: establishing a PC5 RRC connection with a second device; transmitting, to the second device, a first SCI through a PSCCH; transmitting, to the second device, a second SCI and data through a PSSCH related to the PSCCH; determining a PSFCH resource based on an index of a subchannel and a slot related to the PSSCH; based on failure to receive a SL HARQ feedback for the data from the second device based on the PSFCH resource, detecting an RLF for the PC5 RRC connection between the first device and the second device; and performing reselection for the relay device based on detecting the RLF for the PC5 RRC connection.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119 (e), this application claims the benefit ofU.S. Provisional Application No. 63/062,411, filed on Aug. 6, 2020, andKorean Patent Application No. 10-2020-0091914, filed on Jul. 23, 2020,the contents of which are all hereby incorporated by reference herein intheir entirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

This disclosure relates to a wireless communication system.

Related Art

Sidelink (SL) communication is a communication scheme in which a directlink is established between User Equipments (UEs) and the UEs exchangevoice and data directly with each other without intervention of anevolved Node B (eNB). SL communication is under consideration as asolution to the overhead of an eNB caused by rapidly increasing datatraffic. Vehicle-to-everything (V2X) refers to a communicationtechnology through which a vehicle exchanges information with anothervehicle, a pedestrian, an object having an infrastructure (or infra)established therein, and so on. The V2X may be divided into 4 types,such as vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I),vehicle-to-network (V2N), and vehicle-to-pedestrian (V2P). The V2Xcommunication may be provided via a PC5 interface and/or Uu interface.

Meanwhile, as a wider range of communication devices require largercommunication capacities, the need for mobile broadband communicationthat is more enhanced than the existing Radio Access Technology (RAT) isrising. Accordingly, discussions are made on services and user equipment(UE) that are sensitive to reliability and latency. And, a nextgeneration radio access technology that is based on the enhanced mobilebroadband communication, massive Machine Type Communication (MTC),Ultra-Reliable and Low Latency Communication (URLLC), and so on, may bereferred to as a new radio access technology (RAT) or new radio (NR).Herein, the NR may also support vehicle-to-everything (V2X)communication.

FIG. 1 is a drawing for describing V2X communication based on NR,compared to V2X communication based on RAT used before NR. Theembodiment of FIG. 1 may be combined with various embodiments of thepresent disclosure.

Regarding V2X communication, a scheme of providing a safety service,based on a V2X message such as Basic Safety Message (BSM), CooperativeAwareness Message (CAM), and Decentralized Environmental NotificationMessage (DENM) is focused in the discussion on the RAT used before theNR. The V2X message may include position information, dynamicinformation, attribute information, or the like. For example, a UE maytransmit a periodic message type CAM and/or an event triggered messagetype DENM to another UE.

Thereafter, regarding V2X communication, various V2X scenarios areproposed in NR. For example, the various V2X scenarios may includevehicle platooning, advanced driving, extended sensors, remote driving,or the like.

SUMMARY OF THE DISCLOSURE Technical Objects

Meanwhile, a remote UE may perform communication with a base station viaa relay UE. In this case, if a radio link failure (RLF) occurs in thelink between the remote UE and the relay UE, data transmitted by thebase station to the relay UE may not be transferred to the remote UE,and data transmitted by the remote UE to the relay UE may not betransferred to the base station. To solve the above problem, the remoteUE needs to quickly recover the link.

Technical Solutions

In one embodiment, provided is a method for performing wirelesscommunication by a first device. The method may comprise: establishing aPC5 radio resource control (RRC) connection with a second device,wherein the second device is a relay device for relaying communicationbetween the first device and a base station; transmitting, to the seconddevice, a first sidelink control information (SCI) through a physicalsidelink control channel (PSCCH); transmitting, to the second device, asecond SCI and data through a physical sidelink shared channel (PSSCH)related to the PSCCH; determining a physical sidelink feedback channel(PSFCH) resource based on an index of a subchannel and a slot related tothe PSSCH; based on failure to receive a sidelink (SL) hybrid automaticrepeat request (HARQ) feedback for the data from the second device basedon the PSFCH resource, detecting a radio link failure (RLF) for the PC5RRC connection between the first device and the second device; andperforming reselection for the relay device based on detecting the RLFfor the PC5 RRC connection.

In one embodiment, provided is a first device configured to performwireless communication. The first device may comprise: one or morememories storing instructions; one or more transceivers; and one or moreprocessors connected to the one or more memories and the one or moretransceivers. The one or more processors may execute the instructionsto: establish a PC5 radio resource control (RRC) connection with asecond device, wherein the second device is a relay device for relayingcommunication between the first device and a base station; transmit, tothe second device, a first sidelink control information (SCI) through aphysical sidelink control channel (PSCCH); transmit, to the seconddevice, a second SCI and data through a physical sidelink shared channel(PSSCH) related to the PSCCH; determine a physical sidelink feedbackchannel (PSFCH) resource based on an index of a subchannel and a slotrelated to the PSSCH; based on failure to receive a sidelink (SL) hybridautomatic repeat request (HARQ) feedback for the data from the seconddevice based on the PSFCH resource, detect a radio link failure (RLF)for the PC5 RRC connection between the first device and the seconddevice; and perform reselection for the relay device based on detectingthe RLF for the PC5 RRC connection.

Effects of the Disclosure

The user equipment (UE) may efficiently perform relay reselection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing for describing V2X communication based on NR,compared to V2X communication based on RAT used before NR.

FIG. 2 shows a structure of an NR system, in accordance with anembodiment of the present disclosure.

FIG. 3 shows a radio protocol architecture, in accordance with anembodiment of the present disclosure.

FIG. 4 shows a structure of a radio frame of an NR, in accordance withan embodiment of the present disclosure.

FIG. 5 shows a structure of a slot of an NR frame, in accordance with anembodiment of the present disclosure.

FIG. 6 shows an example of a BWP, in accordance with an embodiment ofthe present disclosure.

FIG. 7 shows a UE performing V2X or SL communication, in accordance withan embodiment of the present disclosure.

FIG. 8 shows a procedure of performing V2X or SL communication by a UEbased on a transmission mode, in accordance with an embodiment of thepresent disclosure.

FIG. 9 shows an example of a relay scenario, in accordance with anembodiment of the present disclosure.

FIG. 10 shows a procedure for recovering SL through Uu link, inaccordance with an embodiment of the present disclosure.

FIG. 11 shows a procedure for recovering SL, in accordance with anembodiment of the present disclosure.

FIG. 12 shows a procedure for recovering Uu link through SL, inaccordance with an embodiment of the present disclosure.

FIG. 13 shows a method for a first device to perform wirelesscommunication, in accordance with an embodiment of the presentdisclosure.

FIG. 14 shows a communication system 1, in accordance with an embodimentof the present disclosure.

FIG. 15 shows wireless devices, in accordance with an embodiment of thepresent disclosure.

FIG. 16 shows a signal process circuit for a transmission signal, inaccordance with an embodiment of the present disclosure.

FIG. 17 shows another example of a wireless device, in accordance withan embodiment of the present disclosure.

FIG. 18 shows a hand-held device, in accordance with an embodiment ofthe present disclosure.

FIG. 19 shows a vehicle or an autonomous vehicle, in accordance with anembodiment of the present disclosure.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the present specification, “A or B” may mean “only A”, “only B” or“both A and B.” In other words, in the present specification, “A or B”may be interpreted as “A and/or B”. For example, in the presentspecification, “A, B, or C” may mean “only A”, “only B”, “only C”, or“any combination of A, B, C”.

A slash (/) or comma used in the present specification may mean“and/or”. For example, “A/B” may mean “A and/or B”. Accordingly, “A/B”may mean “only A”, “only B”, or “both A and B”. For example, “A, B, C”may mean “A, B, or C”.

In the present specification, “at least one of A and B” may mean “onlyA”, “only B”, or “both A and B”. In addition, in the presentspecification, the expression “at least one of A or B” or “at least oneof A and/or B” may be interpreted as “at least one of A and B”.

In addition, in the present specification, “at least one of A, B, and C”may mean “only A”, “only B”, “only C”, or “any combination of A, B, andC”. In addition, “at least one of A, B, or C” or “at least one of A, B,and/or C” may mean “at least one of A, B, and C”.

In addition, a parenthesis used in the present specification may mean“for example”. Specifically, when indicated as “control information(PDCCH)”, it may mean that “PDCCH” is proposed as an example of the“control information”. In other words, the “control information” of thepresent specification is not limited to “PDCCH”, and “PDDCH” may beproposed as an example of the “control information”. In addition, whenindicated as “control information (i.e., PDCCH)”, it may also mean that“PDCCH” is proposed as an example of the “control information”.

A technical feature described individually in one figure in the presentspecification may be individually implemented, or may be simultaneouslyimplemented.

The technology described below may be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and so on. TheCDMA may be implemented with a radio technology, such as universalterrestrial radio access (UTRA) or CDMA-2000. The TDMA may beimplemented with a radio technology, such as global system for mobilecommunications (GSM)/general packet ratio service (GPRS)/enhanced datarate for GSM evolution (EDGE). The OFDMA may be implemented with a radiotechnology, such as institute of electrical and electronics engineers(IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA(E-UTRA), and so on. IEEE 802.16m is an evolved version of IEEE 802.16eand provides backward compatibility with a system based on the IEEE802.16e. The UTRA is part of a universal mobile telecommunication system(UMTS). 3rd generation partnership project (3GPP) long term evolution(LTE) is part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTEuses the OFDMA in a downlink and uses the SC-FDMA in an uplink.LTE-advanced (LTE-A) is an evolution of the LTE.

5G NR is a successive technology of LTE-A corresponding to a newClean-slate type mobile communication system having the characteristicsof high performance, low latency, high availability, and so on. 5G NRmay use resources of all spectrum available for usage including lowfrequency bands of less than 1 GHz, middle frequency bands ranging from1 GHz to 10 GHz, high frequency (millimeter waves) of 24 GHz or more,and so on.

For clarity in the description, the following description will mostlyfocus on LTE-A or 5G NR. However, technical features according to anembodiment of the present disclosure will not be limited only to this.

FIG. 2 shows a structure of an NR system, in accordance with anembodiment of the present disclosure. The embodiment of FIG. 2 may becombined with various embodiments of the present disclosure.

Referring to FIG. 2, a next generation-radio access network (NG-RAN) mayinclude a BS 20 providing a UE 10 with a user plane and control planeprotocol termination. For example, the BS 20 may include a nextgeneration-Node B (gNB) and/or an evolved-NodeB (eNB). For example, theUE 10 may be fixed or mobile and may be referred to as other terms, suchas a mobile station (MS), a user terminal (UT), a subscriber station(SS), a mobile terminal (MT), wireless device, and so on. For example,the BS may be referred to as a fixed station which communicates with theUE 10 and may be referred to as other terms, such as a base transceiversystem (BTS), an access point (AP), and so on.

The embodiment of FIG. 2 exemplifies a case where only the gNB isincluded. The BSs 20 may be connected to one another via Xn interface.The BS 20 may be connected to one another via 5th generation (5G) corenetwork (5GC) and NG interface. More specifically, the BSs 20 may beconnected to an access and mobility management function (AMF) 30 viaNG-C interface, and may be connected to a user plane function (UPF) 30via NG-U interface.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (layer 1, L1), a second layer (layer 2,L2), and a third layer (layer 3, L3) based on the lower three layers ofthe open system interconnection (OSI) model that is well-known in thecommunication system. Among them, a physical (PHY) layer belonging tothe first layer provides an information transfer service by using aphysical channel, and a radio resource control (RRC) layer belonging tothe third layer serves to control a radio resource between the UE andthe network. For this, the RRC layer exchanges an RRC message betweenthe UE and the BS.

FIG. 3 shows a radio protocol architecture, in accordance with anembodiment of the present disclosure. The embodiment of FIG. 3 may becombined with various embodiments of the present disclosure.Specifically, (a) of FIG. 3 shows a radio protocol stack of a user planefor Uu communication, and (b) of FIG. 3 shows a radio protocol stack ofa control plane for Uu communication. (c) of FIG. 3 shows a radioprotocol stack of a user plane for SL communication, and (d) of FIG. 3shows a radio protocol stack of a control plane for SL communication.

Referring to FIG. 3, a physical layer provides an upper layer with aninformation transfer service through a physical channel. The physicallayer is connected to a medium access control (MAC) layer which is anupper layer of the physical layer through a transport channel. Data istransferred between the MAC layer and the physical layer through thetransport channel. The transport channel is classified according to howand with what characteristics data is transmitted through a radiointerface.

Between different physical layers, i.e., a physical layer of atransmitter and a physical layer of a receiver, data are transferredthrough the physical channel. The physical channel is modulated using anorthogonal frequency division multiplexing (OFDM) scheme, and utilizestime and frequency as a radio resource.

The MAC layer provides services to a radio link control (RLC) layer,which is a higher layer of the MAC layer, via a logical channel. The MAClayer provides a function of mapping multiple logical channels tomultiple transport channels. The MAC layer also provides a function oflogical channel multiplexing by mapping multiple logical channels to asingle transport channel. The MAC layer provides data transfer servicesover logical channels.

The RLC layer performs concatenation, segmentation, and reassembly ofRadio Link Control Service Data Unit (RLC SDU). In order to ensurediverse quality of service (QoS) required by a radio bearer (RB), theRLC layer provides three types of operation modes, i.e., a transparentmode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM).An AM RLC provides error correction through an automatic repeat request(ARQ).

A radio resource control (RRC) layer is defined only in the controlplane. The RRC layer serves to control the logical channel, thetransport channel, and the physical channel in association withconfiguration, reconfiguration and release of RBs. The RB is a logicalpath provided by the first layer (i.e., the physical layer or the PHYlayer) and the second layer (i.e., a MAC layer, an RLC layer, a packetdata convergence protocol (PDCP) layer, and a service data adaptationprotocol (SDAP) layer) for data delivery between the UE and the network.

Functions of a packet data convergence protocol (PDCP) layer in the userplane include user data delivery, header compression, and ciphering.Functions of a PDCP layer in the control plane include control-planedata delivery and ciphering/integrity protection.

A service data adaptation protocol (SDAP) layer is defined only in auser plane. The SDAP layer performs mapping between a Quality of Service(QoS) flow and a data radio bearer (DRB) and QoS flow ID (QFI) markingin both DL and UL packets.

The configuration of the RB implies a process for specifying a radioprotocol layer and channel properties to provide a particular serviceand for determining respective detailed parameters and operations. TheRB can be classified into two types, i.e., a signaling RB (SRB) and adata RB (DRB). The SRB is used as a path for transmitting an RRC messagein the control plane. The DRB is used as a path for transmitting userdata in the user plane.

When an RRC connection is established between an RRC layer of the UE andan RRC layer of the E-UTRAN, the UE is in an RRC_CONNECTED state, and,otherwise, the UE may be in an RRC_IDLE state. In case of the NR, anRRC_INACTIVE state is additionally defined, and a UE being in theRRC_INACTIVE state may maintain its connection with a core networkwhereas its connection with the BS is released.

Data is transmitted from the network to the UE through a downlinktransport channel. Examples of the downlink transport channel include abroadcast channel (BCH) for transmitting system information and adownlink-shared channel (SCH) for transmitting user traffic or controlmessages. Traffic of downlink multicast or broadcast services or thecontrol messages can be transmitted on the downlink-SCH or an additionaldownlink multicast channel (MCH). Data is transmitted from the UE to thenetwork through an uplink transport channel. Examples of the uplinktransport channel include a random access channel (RACH) fortransmitting an initial control message and an uplink SCH fortransmitting user traffic or control messages.

Examples of logical channels belonging to a higher channel of thetransport channel and mapped onto the transport channels include abroadcast channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH), a multicasttraffic channel (MTCH), etc.

FIG. 4 shows a structure of a radio frame of an NR, in accordance withan embodiment of the present disclosure. The embodiment of FIG. 4 may becombined with various embodiments of the present disclosure.

Referring to FIG. 4, in the NR, a radio frame may be used for performinguplink and downlink transmission. A radio frame has a length of 10 msand may be defined to be configured of two half-frames (HFs). Ahalf-frame may include five lms subframes (SFs). A subframe (SF) may bedivided into one or more slots, and the number of slots within asubframe may be determined in accordance with subcarrier spacing (SCS).Each slot may include 12 or 14 OFDM(A) symbols according to a cyclicprefix (CP).

In case of using a normal CP, each slot may include 14 symbols. In caseof using an extended CP, each slot may include 12 symbols. Herein, asymbol may include an OFDM symbol (or CP-OFDM symbol) and a SingleCarrier-FDMA (SC-FDMA) symbol (or Discrete Fourier Transform-spread-OFDM(DFT-s-OFDM) symbol).

Table 1 shown below represents an example of a number of symbols perslot (N^(slot) _(symb)), a number slots per frame (N^(frame,u) _(slot)),and a number of slots per subframe (N^(subframe,u) _(slot)) inaccordance with an SCS configuration (u), in a case where a normal CP isused.

TABLE 11 SCS (15 * 2^(u)) N^(slot) _(symb) N^(frame,u) _(slot)N^(subframe,u) _(slot)  15 KHz (u = 0) 14 10 1  30 KHz (u = 1) 14 20 2 60 KHz (u = 2) 14 40 4 120 KHz (u = 3) 14 80 8 240 KHz (u = 4) 14 16016

Table 2 shows an example of a number of symbols per slot, a number ofslots per frame, and a number of slots per subframe in accordance withthe SCS, in a case where an extended CP is used.

TABLE 2 SCS (15 * 2^(u)) N^(slot) _(symb) N^(frame,u) _(slot)N^(subframe,u) _(slot) 60 KHz (u = 2) 12 40 4

In an NR system, OFDM(A) numerologies (e.g., SCS, CP length, and so on)between multiple cells being integrate to one UE may be differentlyconfigured. Accordingly, a (absolute time) duration (or section) of atime resource (e.g., subframe, slot or TTI) (collectively referred to asa time unit (TU) for simplicity) being configured of the same number ofsymbols may be differently configured in the integrated cells.

In the NR, multiple numerologies or SCSs for supporting diverse 5Gservices may be supported. For example, in case an SCS is 15 kHz, a widearea of the conventional cellular bands may be supported, and, in casean SCS is 30 kHz/60 kHz a dense-urban, lower latency, wider carrierbandwidth may be supported. In case the SCS is 60 kHz or higher, abandwidth that is greater than 24.25 GHz may be used in order toovercome phase noise.

An NR frequency band may be defined as two different types of frequencyranges. The two different types of frequency ranges may be FR1 and FR2.The values of the frequency ranges may be changed (or varied), and, forexample, the two different types of frequency ranges may be as shownbelow in Table 3. Among the frequency ranges that are used in an NRsystem, FR1 may mean a “sub 6 GHz range”, and FR2 may mean an “above 6GHz range” and may also be referred to as a millimeter wave (mmW).

TABLE 3 Frequency Range Corresponding Subcarrier designation frequencyrange Spacing (SCS) FR1  450 MHz-6000 MHz  15, 30, 60 kHz FR2 24250MHz-52600 MHz 60, 120, 240 kHz

As described above, the values of the frequency ranges in the NR systemmay be changed (or varied). For example, as shown below in Table 4, FR1may include a band within a range of 410 MHz to 7125 MHz. Morespecifically, FR1 may include a frequency band of 6 GHz (or 5850, 5900,5925 MHz, and so on) and higher. For example, a frequency band of 6 GHz(or 5850, 5900, 5925 MHz, and so on) and higher being included in FR1mat include an unlicensed band. The unlicensed band may be used fordiverse purposes, e.g., the unlicensed band for vehicle-specificcommunication (e.g., automated driving).

TABLE 4 Frequency Range Corresponding Subcarrier designation frequencyrange Spacing (SCS) FR1  410 MHz-7125 MHz  15, 30, 60 kHz FR2 24250MHz-52600 MHz 60, 120, 240 kHz

FIG. 5 shows a structure of a slot of an NR frame, in accordance with anembodiment of the present disclosure. The embodiment of FIG. 5 may becombined with various embodiments of the present disclosure.

Referring to FIG. 5, a slot includes a plurality of symbols in a timedomain. For example, in case of a normal CP, one slot may include 14symbols. However, in case of an extended CP, one slot may include 12symbols. Alternatively, in case of a normal CP, one slot may include 7symbols. However, in case of an extended CP, one slot may include 6symbols.

A carrier includes a plurality of subcarriers in a frequency domain. AResource Block (RB) may be defined as a plurality of consecutivesubcarriers (e.g., 12 subcarriers) in the frequency domain. A BandwidthPart (BWP) may be defined as a plurality of consecutive (Physical)Resource Blocks ((P)RBs) in the frequency domain, and the BWP maycorrespond to one numerology (e.g., SCS, CP length, and so on). Acarrier may include a maximum of N number BWPs (e.g., 5 BWPs). Datacommunication may be performed via an activated BWP. Each element may bereferred to as a Resource Element (RE) within a resource grid and onecomplex symbol may be mapped to each element.

Hereinafter, a bandwidth part (BWP) and a carrier will be described.

The BWP may be a set of consecutive physical resource blocks (PRBs) in agiven numerology. The PRB may be selected from consecutive sub-sets ofcommon resource blocks (CRBs) for the given numerology on a givencarrier

For example, the BWP may be at least any one of an active BWP, aninitial BWP, and/or a default BWP. For example, the UE may not monitordownlink radio link quality in a DL BWP other than an active DL BWP on aprimary cell (PCell). For example, the UE may not receive PDCCH,physical downlink shared channel (PDSCH), or channel stateinformation-reference signal (CSI-RS) (excluding RRM) outside the activeDL BWP. For example, the UE may not trigger a channel state information(CSI) report for the inactive DL BWP. For example, the UE may nottransmit physical uplink control channel (PUCCH) or physical uplinkshared channel (PUSCH) outside an active UL BWP. For example, in adownlink case, the initial BWP may be given as a consecutive RB set fora remaining minimum system information (RMSI) control resource set(CORESET) (configured by physical broadcast channel (PBCH)). Forexample, in an uplink case, the initial BWP may be given by systeminformation block (SIB) for a random access procedure. For example, thedefault BWP may be configured by a higher layer. For example, an initialvalue of the default BWP may be an initial DL BWP. For energy saving, ifthe UE fails to detect downlink control information (DCI) during aspecific period, the UE may switch the active BWP of the UE to thedefault BWP.

Meanwhile, the BWP may be defined for SL. The same SL BWP may be used intransmission and reception. For example, a transmitting UE may transmitan SL channel or an SL signal on a specific BWP, and a receiving UE mayreceive the SL channel or the SL signal on the specific BWP. In alicensed carrier, the SL BWP may be defined separately from a Uu BWP,and the SL BWP may have configuration signaling separate from the UuBWP. For example, the UE may receive a configuration for the SL BWP fromthe BS/network. For example, the UE may receive a configuration for theUu BWP from the BS/network. The SL BWP may be (pre-)configured in acarrier with respect to an out-of-coverage NR V2X UE and an RRC_IDLE UE.For the UE in the RRC_CONNECTED mode, at least one SL BWP may beactivated in the carrier.

FIG. 6 shows an example of a BWP, in accordance with an embodiment ofthe present disclosure. The embodiment of FIG. 6 may be combined withvarious embodiments of the present disclosure. It is assumed in theembodiment of FIG. 6 that the number of BWPs is 3.

Referring to FIG. 6, a common resource block (CRB) may be a carrierresource block numbered from one end of a carrier band to the other endthereof. In addition, the PRB may be a resource block numbered withineach BWP. A point A may indicate a common reference point for a resourceblock grid.

The BWP may be configured by a point A, an offset N^(start) _(BWP) fromthe point A, and a bandwidth N^(size) _(BWP). For example, the point Amay be an external reference point of a PRB of a carrier in which asubcarrier 0 of all numerologies (e.g., all numerologies supported by anetwork on that carrier) is aligned. For example, the offset may be aPRB interval between a lowest subcarrier and the point A in a givennumerology. For example, the bandwidth may be the number of PRBs in thegiven numerology.

Hereinafter, V2X or SL communication will be described.

A sidelink synchronization signal (SLSS) may include a primary sidelinksynchronization signal (PSSS) and a secondary sidelink synchronizationsignal (SSSS), as an SL-specific sequence. The PSSS may be referred toas a sidelink primary synchronization signal (S-PSS), and the SSSS maybe referred to as a sidelink secondary synchronization signal (S-SSS).For example, length-127 M-sequences may be used for the S-PSS, andlength-127 gold sequences may be used for the S-SSS. For example, a UEmay use the S-PSS for initial signal detection and for synchronizationacquisition. For example, the UE may use the S-PSS and the S-SSS foracquisition of detailed synchronization and for detection of asynchronization signal ID.

A physical sidelink broadcast channel (PSBCH) may be a (broadcast)channel for transmitting default (system) information which must befirst known by the UE before SL signal transmission/reception. Forexample, the default information may be information related to SLSS, aduplex mode (DM), a time division duplex (TDD) uplink/downlink (UL/DL)configuration, information related to a resource pool, a type of anapplication related to the SLSS, a subframe offset, broadcastinformation, or the like. For example, for evaluation of PSBCHperformance, in NR V2X, a payload size of the PSBCH may be 56 bitsincluding 24-bit cyclic redundancy check (CRC).

The S-PSS, the S-SSS, and the PSBCH may be included in a block format(e.g., SL synchronization signal (SS)/PSBCH block, hereinafter,sidelink-synchronization signal block (S-SSB)) supporting periodicaltransmission. The S-SSB may have the same numerology (i.e., SCS and CPlength) as a physical sidelink control channel (PSCCH)/physical sidelinkshared channel (PSSCH) in a carrier, and a transmission bandwidth mayexist within a (pre-)configured sidelink (SL) BWP. For example, theS-SSB may have a bandwidth of 11 resource blocks (RBs). For example, thePSBCH may exist across 11 RBs. In addition, a frequency position of theS-SSB may be (pre-)configured. Accordingly, the UE does not have toperform hypothesis detection at frequency to discover the S-SSB in thecarrier.

FIG. 7 shows a UE performing V2X or SL communication, in accordance withan embodiment of the present disclosure. The embodiment of FIG. 7 may becombined with various embodiments of the present disclosure.

Referring to FIG. 7, in V2X or SL communication, the term ‘UE’ maygenerally imply a UE of a user. However, if a network equipment such asa BS transmits/receives a signal according to a communication schemebetween UEs, the BS may also be regarded as a sort of the UE. Forexample, a UE 1 may be a first apparatus 100, and a UE 2 may be a secondapparatus 200.

For example, the UE 1 may select a resource unit corresponding to aspecific resource in a resource pool which implies a set of series ofresources. In addition, the UE 1 may transmit an SL signal by using theresource unit. For example, a resource pool in which the UE 1 is capableof transmitting a signal may be configured to the UE 2 which is areceiving UE, and the signal of the UE 1 may be detected in the resourcepool.

Herein, if the UE 1 is within a connectivity range of the BS, the BS mayinform the UE 1 of the resource pool. Otherwise, if the UE 1 is out ofthe connectivity range of the BS, another UE may inform the UE 1 of theresource pool, or the UE 1 may use a pre-configured resource pool.

In general, the resource pool may be configured in unit of a pluralityof resources, and each UE may select a unit of one or a plurality ofresources to use it in SL signal transmission thereof.

Hereinafter, resource allocation in SL will be described.

FIG. 8 shows a procedure of performing V2X or SL communication by a UEbased on a transmission mode, in accordance with an embodiment of thepresent disclosure. The embodiment of FIG. 8 may be combined withvarious embodiments of the present disclosure. In various embodiments ofthe present disclosure, the transmission mode may be called a mode or aresource allocation mode. Hereinafter, for convenience of explanation,in LTE, the transmission mode may be called an LTE transmission mode. InNR, the transmission mode may be called an NR resource allocation mode.

For example, (a) of FIG. 8 shows a UE operation related to an LTEtransmission mode 1 or an LTE transmission mode 3. Alternatively, forexample, (a) of FIG. 8 shows a UE operation related to an NR resourceallocation mode 1. For example, the LTE transmission mode 1 may beapplied to general SL communication, and the LTE transmission mode 3 maybe applied to V2X communication.

For example, (b) of FIG. 8 shows a UE operation related to an LTEtransmission mode 2 or an LTE transmission mode 4. Alternatively, forexample, (b) of FIG. 8 shows a UE operation related to an NR resourceallocation mode 2.

Referring to (a) of FIG. 8, in the LTE transmission mode 1, the LTEtransmission mode 3, or the NR resource allocation mode 1, a BS mayschedule an SL resource to be used by the UE for SL transmission. Forexample, the BS may perform resource scheduling to a UE 1 through aPDCCH (e.g., downlink control information (DCI)) or RRC signaling (e.g.,Configured Grant Type 1 or Configured Grant Type 2), and the UE 1 mayperform V2X or SL communication with respect to a UE 2 according to theresource scheduling. For example, the UE 1 may transmit a sidelinkcontrol information (SCI) to the UE 2 through a physical sidelinkcontrol channel (PSCCH), and thereafter transmit data based on the SCIto the UE 2 through a physical sidelink shared channel (PSSCH).

Referring to (b) of FIG. 8, in the LTE transmission mode 2, the LTEtransmission mode 4, or the NR resource allocation mode 2, the UE maydetermine an SL transmission resource within an SL resource configuredby a BS/network or a pre-configured SL resource. For example, theconfigured SL resource or the pre-configured SL resource may be aresource pool. For example, the UE may autonomously select or schedule aresource for SL transmission. For example, the UE may perform SLcommunication by autonomously selecting a resource within a configuredresource pool. For example, the UE may autonomously select a resourcewithin a selective window by performing a sensing and resource(re)selection procedure. For example, the sensing may be performed inunit of subchannels. In addition, the UE 1 which has autonomouslyselected the resource within the resource pool may transmit the SCI tothe UE 2 through a PSCCH, and thereafter may transmit data based on theSCI to the UE 2 through a PSSCH.

Hereinafter, a sidelink control information (SCI) will be described.

Control information transmitted by a BS to a UE through a PDCCH may bereferred to as downlink control information (DCI), whereas controlinformation transmitted by the UE to another UE through a PSCCH may bereferred to as SCI. For example, the UE may know in advance a startsymbol of the PSCCH and/or the number of symbols of the PSCCH, beforedecoding the PSCCH. For example, the SCI may include SL schedulinginformation. For example, the UE may transmit at least one SCI toanother UE to schedule the PSSCH. For example, one or more SCI formatsmay be defined.

For example, a transmitting UE may transmit the SCI to a receiving UE onthe PSCCH. The receiving UE may decode one SCI to receive the PSSCH fromthe transmitting UE.

For example, the transmitting UE may transmit two consecutive SCIs(e.g., 2-stage SCI) to the receiving UE on the PSCCH and/or the PSSCH.The receiving UE may decode the two consecutive SCIs (e.g., 2-stage SCI)to receive the PSSCH from the transmitting UE. For example, if SCIconfiguration fields are divided into two groups in consideration of a(relatively) high SCI payload size, an SCI including a first SCIconfiguration field group may be referred to as a first SCI or a 1^(st)SCI, and an SCI including a second SCI configuration field group may bereferred to as a second SCI or a 2^(nd) SCI. For example, thetransmitting UE may transmit the first SCI to the receiving UE throughthe PSCCH. For example, the transmitting UE may transmit the second SCIto the receiving UE on the PSCCH and/or the PSSCH. For example, thesecond SCI may be transmitted to the receiving UE through an(independent) PSCCH, or may be transmitted in a piggyback mannertogether with data through the PSSCH. For example, two consecutive SCIsmay also be applied to different transmissions (e.g., unicast,broadcast, or groupcast).

For example, the transmitting UE may transmit the entirety or part ofinformation described below to the receiving UE through the SCI. Herein,for example, the transmitting UE may transmit the entirety or part ofthe information described below to the receiving UE through the firstSCI and/or the second SCI.

-   -   PSSCH and/or PSCCH related resource allocation information,        e.g., the number/positions of time/frequency resources, resource        reservation information (e.g., period), and/or    -   SL CSI report request indicator or SL (L1) RSRP (and/or SL (L1)        RSRQ and/or SL (L1) RSSI) report request indicator, and/or    -   SL CSI transmission indicator (or SL (L1) RSRP (and/or SL (L1)        RSRQ and/or SL (L1) RSSI) information transmission indicator))        (on PSSCH), and/or    -   MCS information, and/or    -   Transmit power information, and/or    -   L1 destination ID information and/or L1 source ID information,        and/or    -   SL HARQ process ID information, and/or    -   New data indicator (NDI) information, and/or    -   Redundancy version (RV) information, and/or    -   (Transmission traffic/packet related) QoS information, e.g.,        priority information, and/or    -   SL CSI-RS transmission indicator or information on the number of        (to-be-transmitted) SL CSI-RS antenna ports, and/or    -   Location information of a transmitting UE or location (or        distance region) information of a target receiving UE (for which        SL HARQ feedback is requested), and/or    -   Reference signal (e.g., DMRS, etc.) related to channel        estimation and/or decoding of data to be transmitted through a        PSSCH, e.g., information related to a pattern of a        (time-frequency) mapping resource of DMRS, rank information,        antenna port index information

For example, the first SCI may include information related to channelsensing. For example, the receiving UE may decode the second SCI byusing a PSSCH DMRS. A polar code used in a PDCCH may be applied to thesecond SCI. For example, in a resource pool, a payload size of the firstSCI may be identical for unicast, groupcast, and broadcast. Afterdecoding the first SCI, the receiving UE does not have to perform blinddecoding of the second SCI. For example, the first SCI may includescheduling information of the second SCI.

Hereinafter, a hybrid automatic repeat request (HARQ) procedure will bedescribed.

In case of SL unicast and groupcast, HARQ feedback and HARQ combining inthe physical layer may be supported. For example, when a receiving UEoperates in a resource allocation mode 1 or 2, the receiving UE mayreceive the PSSCH from a transmitting UE, and the receiving UE maytransmit HARQ feedback for the PSSCH to the transmitting UE by using asidelink feedback control information (SFCI) format through a physicalsidelink feedback channel (PSFCH).

For example, the SL HARQ feedback may be enabled for unicast. In thiscase, in a non-code block group (non-CBG) operation, if the receiving UEdecodes a PSCCH of which a target is the receiving UE and if thereceiving UE successfully decodes a transport block related to thePSCCH, the receiving UE may generate HARQ-ACK. In addition, thereceiving UE may transmit the HARQ-ACK to the transmitting UE.Otherwise, if the receiving UE cannot successfully decode the transportblock after decoding the PSCCH of which the target is the receiving UE,the receiving UE may generate the HARQ-NACK. In addition, the receivingUE may transmit HARQ-NACK to the transmitting UE.

For example, the SL HARQ feedback may be enabled for groupcast. Forexample, in the non-CBG operation, two HARQ feedback options may besupported for groupcast.

(1) Groupcast option 1: After the receiving UE decodes the PSCCH ofwhich the target is the receiving UE, if the receiving UE fails indecoding of a transport block related to the PSCCH, the receiving UE maytransmit HARQ-NACK to the transmitting UE through a PSFCH. Otherwise, ifthe receiving UE decodes the PSCCH of which the target is the receivingUE and if the receiving UE successfully decodes the transport blockrelated to the PSCCH, the receiving UE may not transmit the HARQ-ACK tothe transmitting UE.

(2) Groupcast option 2: After the receiving UE decodes the PSCCH ofwhich the target is the receiving UE, if the receiving UE fails indecoding of the transport block related to the PSCCH, the receiving UEmay transmit HARQ-NACK to the transmitting UE through the PSFCH. Inaddition, if the receiving UE decodes the PSCCH of which the target isthe receiving UE and if the receiving UE successfully decodes thetransport block related to the PSCCH, the receiving UE may transmit theHARQ-ACK to the transmitting UE through the PSFCH.

For example, if the groupcast option 1 is used in the SL HARQ feedback,all UEs performing groupcast communication may share a PSFCH resource.For example, UEs belonging to the same group may transmit HARQ feedbackby using the same PSFCH resource.

For example, if the groupcast option 2 is used in the SL HARQ feedback,each UE performing groupcast communication may use a different PSFCHresource for HARQ feedback transmission. For example, UEs belonging tothe same group may transmit HARQ feedback by using different PSFCHresources.

For example, when the SL HARQ feedback is enabled for groupcast, thereceiving UE may determine whether to transmit the HARQ feedback to thetransmitting UE based on a transmission-reception (TX-RX) distanceand/or RSRP.

For example, in the groupcast option 1, in case of the TX-RXdistance-based HARQ feedback, if the TX-RX distance is less than orequal to a communication range requirement, the receiving UE maytransmit HARQ feedback for the PSSCH to the transmitting UE. Otherwise,if the TX-RX distance is greater than the communication rangerequirement, the receiving UE may not transmit the HARQ feedback for thePSSCH to the transmitting UE. For example, the transmitting UE mayinform the receiving UE of a location of the transmitting UE through SCIrelated to the PSSCH. For example, the SCI related to the PSSCH may besecond SCI. For example, the receiving UE may estimate or obtain theTX-RX distance based on a location of the receiving UE and the locationof the transmitting UE. For example, the receiving UE may decode the SCIrelated to the PSSCH and thus may know the communication rangerequirement used in the PSSCH.

For example, in case of the resource allocation mode 1, a time (offset)between the PSFCH and the PSSCH may be configured or pre-configured. Incase of unicast and groupcast, if retransmission is necessary on SL,this may be indicated to a BS by an in-coverage UE which uses the PUCCH.The transmitting UE may transmit an indication to a serving BS of thetransmitting UE in a form of scheduling request (SR)/buffer statusreport (BSR), not a form of HARQ ACK/NACK. In addition, even if the BSdoes not receive the indication, the BS may schedule an SLretransmission resource to the UE. For example, in case of the resourceallocation mode 2, a time (offset) between the PSFCH and the PSSCH maybe configured or pre-configured.

For example, from a perspective of UE transmission in a carrier, TDMbetween the PSCCH/PSSCH and the PSFCH may be allowed for a PSFCH formatfor SL in a slot. For example, a sequence-based PSFCH format having asingle symbol may be supported. Herein, the single symbol may not an AGCduration. For example, the sequence-based PSFCH format may be applied tounicast and groupcast.

For example, in a slot related to a resource pool, a PSFCH resource maybe configured periodically as N slot durations, or may bepre-configured. For example, N may be configured as one or more valuesgreater than or equal to 1. For example, N may be 1, 2, or 4. Forexample, HARQ feedback for transmission in a specific resource pool maybe transmitted only through a PSFCH on the specific resource pool.

For example, if the transmitting UE transmits the PSSCH to the receivingUE across a slot #X to a slot #N, the receiving UE may transmit HARQfeedback for the PSSCH to the transmitting UE in a slot #(N+A). Forexample, the slot #(N+A) may include a PSFCH resource. Herein, forexample, A may be a smallest integer greater than or equal to K. Forexample, K may be the number of logical slots. In this case, K may bethe number of slots in a resource pool. Alternatively, for example, Kmay be the number of physical slots. In this case, K may be the numberof slots inside or outside the resource pool.

For example, if the receiving UE transmits HARQ feedback on a PSFCHresource in response to one PSSCH transmitted by the transmitting UE tothe receiving UE, the receiving UE may determine a frequency domainand/or code domain of the PSFCH resource based on an implicit mechanismin a configured resource pool. For example, the receiving UE maydetermine the frequency domain and/or code domain of the PSFCH resource,based on at least one of a slot index related to PSCCH/PSSCH/PSFCH, asub-channel related to PSCCH/PSSCH, and/or an identifier for identifyingeach receiving UE in a group for HARQ feedback based on the groupcastoption 2. Additionally/alternatively, for example, the receiving UE maydetermine the frequency domain and/or code domain of the PSFCH resource,based on at least one of SL RSRP, SINR, L1 source ID, and/or locationinformation.

For example, if HARQ feedback transmission through the PSFCH of the UEand HARQ feedback reception through the PSFCH overlap, the UE may selectany one of HARQ feedback transmission through the PSFCH and HARQfeedback reception through the PSFCH based on a priority rule. Forexample, the priority rule may be based on at least priority indicationof the related PSCCH/PSSCH.

For example, if HARQ feedback transmission of a UE through a PSFCH for aplurality of UEs overlaps, the UE may select specific HARQ feedbacktransmission based on the priority rule. For example, the priority rulemay be based on at least priority indication of the related PSCCH/PSSCH.

Hereinafter, SL radio link monitoring (RLM) will be described.

In case of AS-level link management of unicast, SL radio link monitoring(RLM) and/or radio link failure (RLF) declaration may be supported. Incase of RLC acknowledged mode (AM) in SL unicast, the RLF declarationmay be triggered by an indication from RLC indicating that the maximumnumber of retransmissions has been reached. An AS-level link status(e.g., failure) may need to be informed to a higher layer. Unlike theRLM procedure for unicast, a groupcast-related RLM design may not beconsidered. The RLM and/or RLF declarations may not be necessary betweengroup members for groupcast.

For example, a transmitting UE may transmit a reference signal to areceiving UE, and the receiving UE may perform SL RLM by using thereference signal. For example, the receiving UE may declare SL RLF byusing the reference signal. For example, the reference signal may bereferred to as an SL reference signal.

Hereinafter, SL measurement and reporting will be described.

For the purpose of QoS prediction, initial transmission parametersetting, link adaptation, link management, admission control, or thelike, SL measurement and reporting (e.g., RSRP, RSRQ) between UEs may beconsidered in SL. For example, a receiving UE may receive a referencesignal from a transmitting UE, and the receiving UE may measure achannel state for the transmitting UE based on the reference signal. Inaddition, the receiving UE may report channel state information (CSI) tothe transmitting UE. SL-related measurement and reporting may includemeasurement and reporting of CBR and reporting of location information.Examples of channel status information (CSI) for V2X may include achannel quality indicator (CQI), a precoding matrix index (PM), a rankindicator (RI), reference signal received power (RSRP), reference signalreceived quality (RSRQ), pathgain/pathloss, a sounding reference symbol(SRS) resource indicator (SRI), a SRI-RS resource indicator (CRI), aninterference condition, a vehicle motion, or the like. In case ofunicast communication, CQI, RI, and PMI or some of them may be supportedin a non-subband-based aperiodic CSI report under the assumption of fouror less antenna ports. A CSI procedure may not be dependent on astandalone reference signal (RS). A CSI report may be activated ordeactivated based on a configuration.

For example, the transmitting UE may transmit CSI-RS to the receivingUE, and the receiving UE may measure CQI or RI based on the CSI-RS. Forexample, the CSI-RS may be referred to as SL CSI-RS. For example, theCSI-RS may be confined within PSSCH transmission. For example, thetransmitting UE may perform transmission to the receiving UE byincluding the CSI-RS on the PSSCH.

Meanwhile, in case of performing SL relay, a remote UE and a relay UEmay detect each other and may set up PC5 connection(s) each otherthrough PC5-signaling. For example, the remote UE and the relay UE mayset up PC5-RRC connection(s). In this way, a state in which the PC5connection and/or the PC5-RRC connection is set up between the relay UEand the remote UE may be referred to as a linked state.

FIG. 9 shows an example of a relay scenario, in accordance with anembodiment of the present disclosure. The embodiment of FIG. 9 may becombined with various embodiments of the present disclosure.

Meanwhile, in current operation, if an RLF occurs in the MAC layer orthe RLC layer while the PC5-RRC connection is established, the UEimmediately releases the PC5-RRC connection. That is, in case the RLFoccurs in SL, there is no link recovery procedure.

From a network point of view, it is like a relay Uu link, and like ageneral Uu link, if there is a problem in the link, a quick recoveryprocedure may be required.

Conversely, even if an RLF occurs in Uu link, the remote UE may informthe relay UE that an RLF of a master cell group (MCG) has occurredthrough SL, and a procedure for quickly recovering the Uu link based onthis may also be considered.

FIG. 10 shows a procedure for recovering SL through Uu link, inaccordance with an embodiment of the present disclosure. The embodimentof FIG. 10 may be combined with various embodiments of the presentdisclosure. Some of the procedures of FIG. 10 may be omitted, and theillustrated procedures do not necessarily have to be performed as awhole.

Referring to FIG. 10, a remote UE may be configured with an SL-RLF timerfor selecting a relay UE. For example, the remote UE may receive aconfiguration related to a timer for selecting the relay UE, from thebase station. For example, the remote UE may receive a configurationrelated to a timer for selecting the relay UE, from other UE(s). Forexample, the remote UE may perform recovery operation while theconfigured timer operates.

In step S1010, the remote UE may detect a SL RLF. For example, if atleast one of the following conditions is satisfied, the remote UE maydetect the SL RLF. For example, if the SL RLF occurs, the remote UE mayimmediately start the SL-RLF timer.

-   -   if T400 expires, and/or    -   if integrity check failure is detected in the PDCP layer, e.g.,        if information related to integrity check failure is transmitted        from the PDCP layer to the RRC layer, and/or    -   if the number of transmissions of retransmissions in the RLC        layer reaches a maximum value, and/or    -   if the number of transmissions of HARQ retransmissions in the        MAC layer reaches a maximum value, and/or    -   if the number of out-of-syncs of sync transmitted from the PHY        layer reaches a certain number;

Specifically, for example, the remote UE may transmit the first SCI tothe relay UE through the PSCCH, and the remote UE may transmit thesecond SCI and the MAC PDU to the relay UE through the PSSCH. In thiscase, for example, the remote UE and/or the relay UE may determine thePSFCH resource related to the PSSCH based on an index of a subchanneland a slot related to the PSSCH. In this case, for example, if theremote UE fails to detect HARQ feedback for the MAC PDU on the PSFCHresource, the remote UE may retransmit the MAC PDU through the PSSCH.For example, if the number of (re)transmissions for the MAC PDU reachesa threshold configured for the remote UE, the remote UE may detect theSL RLF for an RRC connection between the remote UE and the relay UE.That is, for example, if the number of times the remote UE fails todetect HARQ feedback for the MAC PDU reaches a threshold configured forthe remote UE, the remote UE may detect the SL RLF for an RRC connectionbetween the remote UE and the relay UE. For example, if the number oftimes the remote UE consecutively fails to detect HARQ feedback for theMAC PDU reaches a threshold configured for the remote UE, the remote UEmay detect the SL RLF for an RRC connection between the remote UE andthe relay UE.

Specifically, for example, the remote UE may fail to detect SL HARQfeedback for the MAC PDU in the following cases. The cases describedbelow are only examples, and the technical idea of the presentdisclosure is not limited to the following examples.

(1) Case 1—Problem of Uu Link Between Base Station and Relay UE

For example, the remote UE may transmit data to the relay UE through thePSSCH, and the relay UE may relay the data to the base station throughthe PUSCH. In this case, the base station may not be able to receive thedata from the relay UE due to a link problem (e.g., deterioration oflink quality) between the base station and the relay UE. For thisreason, the relay UE may not receive HARQ feedback for the data from thebase station, and the relay UE may not transmit SL HARQ feedback for thedata to the remote UE. In this case, even though the remote UE hastransmitted the data to the relay UE through the PSSCH, the remote UEmay fail to detect SL HARQ feedback for the data on the PSFCH resourcerelated to the PSSCH.

(2) Case 2—HALF Duplex Problem

For example, the remote UE may transmit first data to the relay UEthrough the PSSCH, and the relay UE may transmit second data to the basestation through the PUSCH. Herein, the first data and the second datamay be different data. In the above case, if a time domain of the PSSCHresource and a time domain of the PUSCH resource partially or fullyoverlap, the relay UE may fail to detect the first data due to the HALFduplex problem (e.g., the problem of not being able to transmit andreceive simultaneously at the same time), and the relay UE may nottransmit SL HARQ feedback for the first data to the remote UE. In thiscase, even though the remote UE has transmitted the first data to therelay UE through the PSSCH, the remote UE may fail to detect SL HARQfeedback for the first data on the PSFCH resource related to the PSSCH.

(3) Case 3—SL Omission Due to Prioritization Between UL and SL

For example, the remote UE may transmit data to the relay UE through thePSSCH, and the relay UE may generate SL HARQ feedback for the data.Herein, for example, it is assumed that the relay UE has UL information(e.g., data, control information, HARQ feedback, etc.) to be transmittedbased on a UL resource (e.g., PUCCH resource or PUSCH resource), and theUL resource partially or fully overlaps with the PSFCH resource in whichSL HARQ feedback is transmitted in a time domain. In this case, if therelay UE does not have capability of transmitting the SL HARQ feedbackand the UL information at the same time, the relay UE may omit any onetransmission. For example, in the above-described case, due toprioritization between UL and SL, the relay UE may not transmit SL HARQfeedback for the data to the remote UE. In this case, even though theremote UE has transmitted the data to the relay UE through the PSSCH,the remote UE may fail to detect SL HARQ feedback for the data on thePSFCH resource related to the PSSCH.

For example, the remote UE may suspend all SL transmissions. Forexample, in case of SRB, the remote UE may suspend at least one ofSL-SRB0, SL-SRB1, SL-SRB2, and/or SL-SRB3. For example, in case of DRB,the remote UE may suspend SL-DRB(s).

In the embodiment of FIG. 10, SL may be recovered via a network. Forexample, in the embodiment of FIG. 10, the relay UE may be re-selectedby a network.

In step S1020, the remote UE may inform the network of the occurrence ofthe SL RLF. For example, if the remote UE is in an RRC_CONNECTED state,the remote UE may immediately inform the network of the occurrence ofthe SL RLF. For example, if the remote UE is in an RRC_IDLE state, theremote UE may inform the network of the occurrence of the SL RLF afterperforming random access (RA).

For example, a message informing the network of the SL RLF may beincluded in Sidelink UE information NR message (SidelinkUEInformationNR,RRC message) or may be defined as a separate sidelink failureinformation message. For example, the transmitted message may include atleast one of a specific cause of the SL-RLF, a list of relay UEs for SLrecovery, and/or measurement information (e.g., measurement report)regarding the relay UEs.

For example, the RLF cause may be an expiration of a timer. For example,the RLF cause may include information representing an RLF-related layer(e.g., PDCP, RLC, MAC, or PHY).

For example, the measurement information (e.g., measurement report) mayinclude RSRP or RSRQ measurement results of a serving relay UE. Forexample, the measurement information (e.g., measurement report) mayinclude RSRP or RSRQ measurement results of a non-serving relay UE.

In step S1030, the network receiving the measurement information for therelay UE and/or the SL RLF may transmit a message for SL recovery to theremote UE. Specifically, for example, the message may include listinformation of UE IDs or UE ID information of relay UE(s) connected tothe same network as the remote UE. For example, measurement informationfor relay UE(s) may be used for selection of a new relay UE.Specifically, for example, the relay UEs may be composed of relay UEsconnected to the same network and configured in the same serving cell asthe remote UE.

For example, if the SL-RLF timer expires while the remote UE does notreceive the message for SL recovery in step S1030, the remote UE mayimmediately release the PC5-RRC connection. Additionally, for example,the remote UE may inform the network of the cause of the SL-RLF and/orthe expiration of the timer through a message (e.g., sidelink UEinformation NR message).

In step S1040, the remote UE may establish a PC5-RRC connection with anewly allocated relay UE, and the remote UE may resume transmissionthrough the suspended SL-SRB and SL-DRB.

FIG. 11 shows a procedure for recovering SL, in accordance with anembodiment of the present disclosure. The embodiment of FIG. 11 may becombined with various embodiments of the present disclosure. Some of theprocedures of FIG. 11 may be omitted, and the illustrated procedures donot necessarily have to be performed as a whole.

Referring to FIG. 11, a remote UE may be configured with an SL-RLF timerfor selecting a relay UE. For example, the remote UE may receive aconfiguration related to a timer for selecting the relay UE, from thebase station. For example, the remote UE may receive a configurationrelated to a timer for selecting the relay UE, from other UE(s). Forexample, the remote UE may perform recovery operation while theconfigured timer operates.

In step S1110, the remote UE may detect SL RLF. For example, if at leastone of the following conditions is satisfied, the remote UE may detectthe SL RLF. For example, if the SL RLF occurs, the remote UE mayimmediately start the SL-RLF timer.

-   -   if T400 expires, and/or    -   if integrity check failure is detected in the PDCP layer, e.g.,        if information related to integrity check failure is transmitted        from the PDCP layer to the RRC layer, and/or    -   if the number of transmissions of retransmissions in the RLC        layer reaches a maximum value, and/or    -   if the number of transmissions of HARQ retransmissions in the        MAC layer reaches a maximum value, and/or    -   if the number of out-of-syncs of sync transmitted from the PHY        layer reaches a certain number;

Specifically, for example, the remote UE may transmit the first SCI tothe relay UE through the PSCCH, and the remote UE may transmit thesecond SCI and the MAC PDU to the relay UE through the PSSCH. In thiscase, for example, the remote UE and/or the relay UE may determine thePSFCH resource related to the PSSCH based on an index of a subchanneland a slot related to the PSSCH. In this case, for example, if theremote UE fails to detect HARQ feedback for the MAC PDU on the PSFCHresource, the remote UE may retransmit the MAC PDU through the PSSCH.For example, if the number of (re)transmissions for the MAC PDU reachesa threshold configured for the remote UE, the remote UE may detect theSL RLF for an RRC connection between the remote UE and the relay UE.That is, for example, if the number of times the remote UE fails todetect HARQ feedback for the MAC PDU reaches a threshold configured forthe remote UE, the remote UE may detect the SL RLF for an RRC connectionbetween the remote UE and the relay UE. For example, if the number oftimes the remote UE consecutively fails to detect HARQ feedback for theMAC PDU reaches a threshold configured for the remote UE, the remote UEmay detect the SL RLF for an RRC connection between the remote UE andthe relay UE.

Specifically, for example, the remote UE may fail to detect SL HARQfeedback for the MAC PDU in the following cases. The cases describedbelow are only examples, and the technical idea of the presentdisclosure is not limited to the following examples.

In the embodiment of FIG. 11, SL may be recovered via the remote UE. Forexample, in the embodiment of FIG. 11, the relay UE may be re-selectedby the remote UE.

In step S1120, if the remote UE detects the SL RLF, the remote UE mayperform reselection for the relay UE. For example, if the remote UEdetects the SL RLF, the remote UE may trigger reselection for the relayUE. For example, the remote UE may perform a procedure for detecting(new) relay UE(s).

For example, in a procedure for detecting (new) relay UE(s), the remoteUE may preferentially search for suitable relay UE(s) among relay UEsalready known. For example, in a procedure for detecting (new) relayUE(s), the remote UE may preferentially search for suitable relay UE(s)among relay UEs already known based on measurement.

For example, if there is no suitable relay UE among relay UEs alreadyknown, the remote UE may perform a discovery procedure to detect otherrelay UE(s).

For example, the remote UE may receive a configuration (i.e., threshold)for suitable relay UE(s) from the network. For example, if an RSRP valueand/or an RSRQ value of a relay UE exceeds the configured threshold, theremote UE may determine that the relay UE is a suitable relay UE.

In step S1130, in case the remote UE detects a relay UE, the remote UEmay transmit an UE ID of the detected relay UE through a message (e.g.,sidelink UE information NR message) to the network. For example, in casethe remote UE detects no suitable relay UE, if the timer expires, theremote UE may release the PC5-RRC connection. Additionally, for example,the remote UE may inform the network of the cause of the SL-RLF and/orthe failure of detection of the relay UE through a message (e.g.,sidelink UE information NR message).

For example, the remote UE may establish a PC5-RRC connection with thenewly detected relay UE, and the remote UE may resume transmissionthrough the suspended SL-SRB and SL-DRB.

FIG. 12 shows a procedure for recovering Uu link through SL, inaccordance with an embodiment of the present disclosure. The embodimentof FIG. 12 may be combined with various embodiments of the presentdisclosure. Some of the procedures of FIG. 12 may be omitted, and theillustrated procedures do not necessarily have to be performed as awhole.

Referring to FIG. 12, in step S1210, a remote UE may detect RLF of theUu link. For example, RLF conditions of the Uu link may be the same asRLF determination conditions of the existing Uu link. For example, theremote UE may detect the RLF of the Uu link based on proceduresdescribed in 5.3.10 of 3GPP TS 38.331 V16.0.0.

For example, if the RLF of the Uu link occurs in the remote UE, theremote UE may suspend transmission of all Uu links. For example, if theRLF of the Uu link occurs in the remote UE, the remote UE may suspendtransmission of all Uu links except for SRB0. Herein, for example, SRB1,SRB2 and DRB may be suspended. Additionally, for example, the remote UEmay not perform an RRC re-establishment procedure for the Uu link. Forconvenience of description, the RLF of the Uu link may be referred to asUu RLF.

For example, the remote UE may perform a procedure for recovering the Uulink by exchanging information for recovery of the Uu link with thenetwork through PC5 link. This may be possible via relay UE(s) havingPC5 connection(s) with the remote UE. For example, if there is no relayUE connected to the remote UE through PC5, the remote UE may detect newrelay UE(s) through a discovery procedure.

In step S1220, the remote UE may inform the network of the occurrence ofthe Uu RLF through SL. For example, a message informing the network ofthe Uu RLF may be included in a Sidelink UE information NR message ormay be defined as a separate sidelink failure information message. Forexample, the transmitted message may include MCG failure information.

In step S1230, the network may transmit a message for recovering the Uulink of the remote UE to the relay UE. For example, the message forrecovering the Uu link of the remote UE may be transmitted to the remoteUE via the relay UE. For example, the message may includereconfiguration with sync information for performing PCell change (i.e.,handover).

In step S1240, the remote UE may receive a message for the recoveryprocedure via the relay UE (e.g., through the PC5 link), and the remoteUE may change the PCell, and the remote UE may resume transmissionthrough the suspended SRB/DRB.

For example, various embodiments of the present disclosure may beapplied to a situation in which the remote UE is connected to both theSL and the Uu link (e.g., a situation in which reliability is increasedthrough duplication). For example, various embodiments of the presentdisclosure may be applied to a case in which an RLF occurs in a physicallayer due to out-of-sync occurring in transmission through the SSB ofthe Uu link, even though a UE transmits/receives data through the SL butdoes not transmit/receive data through the Uu link.

According to various embodiments of the present disclosure, if the SLRLF or the Uu RLF occurs, the corresponding link may be quicklyrecovered. For example, if the remote UE fails to detect SL HARQfeedback from the relay UE even though the remote UE has (consecutively)transmitted data N times to the relay UE, the remote UE may determinethat an RLF has occurred in the link between the remote UE and the relayUE. Herein, N may be a positive integer value configured for the remoteUE. In this case, the remote UE may trigger a procedure for reselectinga new relay UE. Therefore, the remote UE can quickly select a new relayUE, and communication between the remote UE and the base station can bequickly resumed via the new relay UE.

FIG. 13 shows a method for a first device to perform wirelesscommunication, in accordance with an embodiment of the presentdisclosure. The embodiment of FIG. 13 may be combined with variousembodiments of the present disclosure.

Referring to FIG. 13, in step S1310, the first device may establish aPC5 radio resource control (RRC) connection with a second device,wherein the second device is a relay device for relaying communicationbetween the first device and a base station. In step S1320, the firstdevice may transmit, to the second device, a first sidelink controlinformation (SCI) through a physical sidelink control channel (PSCCH).In step S1330, the first device may transmit, to the second device, asecond SCI and data through a physical sidelink shared channel (PSSCH)related to the PSCCH. In step S1340, the first device may determine aphysical sidelink feedback channel (PSFCH) resource based on an index ofa subchannel and a slot related to the PSSCH. In step S1350, the firstdevice may detect a radio link failure (RLF) for the PC5 RRC connectionbetween the first device and the second device based on failure toreceive a sidelink (SL) hybrid automatic repeat request (HARQ) feedbackfor the data from the second device based on the PSFCH resource. In stepS1360, the first device may perform reselection for the relay devicebased on detecting the RLF for the PC5 RRC connection.

For example, based on a number of failure to receive the SL HARQfeedback reaching a first threshold, the RLF for the PC5 RRC connectionmay be detected by the first device. For example, based on a number ofout-of-syncs (OOSs) transferred from a physical layer of the firstdevice to an RRC layer of the first device reaching a second threshold,the RLF for the PC5 RRC connection may be detected by the first device.For example, based on detecting an integrity check failure, the RLF forthe PC5 RRC connection may be detected by the first device.

For example, the first device may be allowed to perform reselection forthe relay device while a timer is running. For example, the timer may beconfigured for the first device.

For example, based on detecting the RLF for the PC5 RRC connection, asignaling radio bearer (SRB) and a data radio bearer (DRB) related tothe PC5 RRC connection between the first device and the second devicemay be suspended.

For example, based on detecting the RLF for the PC5 RRC connection,reselection for the relay device may be triggered by the first device.

Additionally, for example, the first device may transmit a messageincluding (i) a cause of the RLF, (ii) a list of candidate relaydevices, and (iii) a result of channel measurement for the candidaterelay devices, to the base station, based on detecting the RLF for thePC5 RRC connection. For example, the cause of the RLF may include atleast one of information representing that the RLF is related toexpiration of a timer, information representing that the RLF is relatedto a packet data convergence protocol (PDCP) layer, informationrepresenting that the RLF is related to a radio link control (RLC)layer, information representing that the RLF is related to a mediumaccess control (MAC) layer, or information representing that the RLF isrelated to a physical layer.

For example, the reselection for the relay device may comprise:performing channel measurement for at least one candidate relay device;and selecting a third device for which a result of the channelmeasurement exceeds a third threshold from among the at least onecandidate relay device as the relay device. Additionally, for example,the first device may transmit a message including UE ID of the thirddevice to the base station. Additionally, for example, the first devicemay establish the PC5 RRC connection with the third device. For example,the third threshold may be configured by the base station to the firstdevice.

Additionally, for example, the first device may transmit a messageincluding (i) information representing the failure of the reselectionand (ii) a cause of the RLF, to the base station, based on failure ofthe reselection for the relay device. For example, the cause of the RLFmay include at least one of information representing that the RLF isrelated to expiration of a timer, information representing that the RLFis related to a packet data convergence protocol (PDCP) layer,information representing that the RLF is related to a radio link control(RLC) layer, information representing that the RLF is related to amedium access control (MAC) layer, or information representing that theRLF is related to a physical layer.

The proposed method may be applied to an apparatus according to variousembodiments of the present disclosure. First, the processor (102) of thefirst device (100) may establish a PC5 radio resource control (RRC)connection with a second device, wherein the second device is a relaydevice for relaying communication between the first device and a basestation. In addition, the processor (102) of the first device (100) maycontrol the transceiver (106) to transmit, to the second device, a firstsidelink control information (SCI) through a physical sidelink controlchannel (PSCCH). In addition, the processor (102) of the first device(100) may control the transceiver (106) to transmit, to the seconddevice, a second SCI and data through a physical sidelink shared channel(PSSCH) related to the PSCCH. In addition, the processor (102) of thefirst device (100) may determine a physical sidelink feedback channel(PSFCH) resource based on an index of a subchannel and a slot related tothe PSSCH. In addition, the processor (102) of the first device (100)may detect a radio link failure (RLF) for the PC5 RRC connection betweenthe first device and the second device based on failure to receive asidelink (SL) hybrid automatic repeat request (HARQ) feedback for thedata from the second device based on the PSFCH resource. In addition,the processor (102) of the first device (100) may perform reselectionfor the relay device based on detecting the RLF for the PC5 RRCconnection.

According to an embodiment of the present disclosure, a first deviceconfigured to perform wireless communication may be provided. Forexample, the first device may comprise: one or more memories storinginstructions; one or more transceivers; and one or more processorsconnected to the one or more memories and the one or more transceivers.For example, the one or more processors may execute the instructions to:establish a PC5 radio resource control (RRC) connection with a seconddevice, wherein the second device is a relay device for relayingcommunication between the first device and a base station; transmit, tothe second device, a first sidelink control information (SCI) through aphysical sidelink control channel (PSCCH); transmit, to the seconddevice, a second SCI and data through a physical sidelink shared channel(PSSCH) related to the PSCCH; determine a physical sidelink feedbackchannel (PSFCH) resource based on an index of a subchannel and a slotrelated to the PSSCH; based on failure to receive a sidelink (SL) hybridautomatic repeat request (HARQ) feedback for the data from the seconddevice based on the PSFCH resource, detect a radio link failure (RLF)for the PC5 RRC connection between the first device and the seconddevice; and perform reselection for the relay device based on detectingthe RLF for the PC5 RRC connection.

For example, based on a number of failure to receive the SL HARQfeedback reaching a first threshold, the RLF for the PC5 RRC connectionmay be detected by the first device.

According to an embodiment of the present disclosure, an apparatusconfigured to control a first user equipment (UE) may be provided. Forexample, the apparatus may comprise: one or more processors; and one ormore memories operably connected to the one or more processors andstoring instructions. For example, the one or more processors mayexecute the instructions to: establish a PC5 radio resource control(RRC) connection with a second UE, wherein the second UE is a relay UEfor relaying communication between the first UE and a base station;transmit, to the second UE, a first sidelink control information (SCI)through a physical sidelink control channel (PSCCH); transmit, to thesecond UE, a second SCI and data through a physical sidelink sharedchannel (PSSCH) related to the PSCCH; determine a physical sidelinkfeedback channel (PSFCH) resource based on an index of a subchannel anda slot related to the PSSCH; based on failure to receive a sidelink (SL)hybrid automatic repeat request (HARQ) feedback for the data from thesecond UE based on the PSFCH resource, detect a radio link failure (RLF)for the PC5 RRC connection between the first UE and the second UE; andperform reselection for the relay UE based on detecting the RLF for thePC5 RRC connection.

For example, based on a number of failure to receive the SL HARQfeedback reaching a first threshold, the RLF for the PC5 RRC connectionmay be detected by the first UE.

According to an embodiment of the present disclosure, a non-transitorycomputer-readable storage medium storing instructions may be provided.For example, the instructions, when executed, cause a first device to:establish a PC5 radio resource control (RRC) connection with a seconddevice, wherein the second device is a relay device for relayingcommunication between the first device and a base station; transmit, tothe second device, a first sidelink control information (SCI) through aphysical sidelink control channel (PSCCH); transmit, to the seconddevice, a second SCI and data through a physical sidelink shared channel(PSSCH) related to the PSCCH; determine a physical sidelink feedbackchannel (PSFCH) resource based on an index of a subchannel and a slotrelated to the PSSCH; based on failure to receive a sidelink (SL) hybridautomatic repeat request (HARQ) feedback for the data from the seconddevice based on the PSFCH resource, detect a radio link failure (RLF)for the PC5 RRC connection between the first device and the seconddevice; and perform reselection for the relay device based on detectingthe RLF for the PC5 RRC connection.

Various embodiments of the present disclosure may be combined with eachother.

Hereinafter, device(s) to which various embodiments of the presentdisclosure can be applied will be described.

The various descriptions, functions, procedures, proposals, methods,and/or operational flowcharts of the present disclosure described inthis document may be applied to, without being limited to, a variety offields requiring wireless communication/connection (e.g., 5G) betweendevices.

Hereinafter, a description will be given in more detail with referenceto the drawings. In the following drawings/description, the samereference symbols may denote the same or corresponding hardware blocks,software blocks, or functional blocks unless described otherwise.

FIG. 14 shows a communication system 1, in accordance with an embodimentof the present disclosure.

Referring to FIG. 14, a communication system 1 to which variousembodiments of the present disclosure are applied includes wirelessdevices, Base Stations (BSs), and a network. Herein, the wirelessdevices represent devices performing communication using Radio AccessTechnology (RAT) (e.g., 5G New RAT (NR)) or Long-Term Evolution (LTE))and may be referred to as communication/radio/5G devices. The wirelessdevices may include, without being limited to, a robot 100 a, vehicles100 b-1 and 100 b-2, an eXtended Reality (XR) device 100 c, a hand-helddevice 100 d, a home appliance 100 e, an Internet of Things (IoT) device100 f, and an Artificial Intelligence (AI) device/server 400. Forexample, the vehicles may include a vehicle having a wirelesscommunication function, an autonomous vehicle, and a vehicle capable ofperforming communication between vehicles. Herein, the vehicles mayinclude an Unmanned Aerial Vehicle (UAV) (e.g., a drone). The XR devicemay include an Augmented Reality (AR)/Virtual Reality (VR)/Mixed Reality(MR) device and may be implemented in the form of a Head-Mounted Device(HMD), a Head-Up Display (HUD) mounted in a vehicle, a television, asmartphone, a computer, a wearable device, a home appliance device, adigital signage, a vehicle, a robot, etc. The hand-held device mayinclude a smartphone, a smartpad, a wearable device (e.g., a smartwatchor a smartglasses), and a computer (e.g., a notebook). The homeappliance may include a TV, a refrigerator, and a washing machine. TheIoT device may include a sensor and a smartmeter. For example, the BSsand the network may be implemented as wireless devices and a specificwireless device 200 a may operate as a BS/network node with respect toother wireless devices.

Here, wireless communication technology implemented in wireless devices100 a to 100 f of the present disclosure may include Narrowband Internetof Things for low-power communication in addition to LTE, NR, and 6G. Inthis case, for example, NB-IoT technology may be an example of Low PowerWide Area Network (LPWAN) technology and may be implemented as standardssuch as LTE Cat NB1, and/or LTE Cat NB2, and is not limited to the namedescribed above. Additionally or alternatively, the wirelesscommunication technology implemented in the wireless devices 100 a to100 f of the present disclosure may perform communication based on LTE-Mtechnology. In this case, as an example, the LTE-M technology may be anexample of the LPWAN and may be called by various names includingenhanced Machine Type Communication (eMTC), and the like. For example,the LTE-M technology may be implemented as at least any one of variousstandards such as 1) LTE CAT 0, 2) LTE Cat M1, 3) LTE Cat M2, 4) LTEnon-Bandwidth Limited (non-BL), 5) LTE-MTC, 6) LTE Machine TypeCommunication, and/or 7) LTE M, and is not limited to the name describedabove. Additionally or alternatively, the wireless communicationtechnology implemented in the wireless devices 100 a to 100 f of thepresent disclosure may include at least one of Bluetooth, Low Power WideArea Network (LPWAN), and ZigBee considering the low-powercommunication, and is not limited to the name described above. As anexample, the ZigBee technology may generate personal area networks (PAN)related to small/low-power digital communication based on variousstandards including IEEE 802.15.4, and the like, and may be called byvarious names.

The wireless devices 100 a to 100 f may be connected to the network 300via the BSs 200. An AI technology may be applied to the wireless devices100 a to 100 f and the wireless devices 100 a to 100 f may be connectedto the AI server 400 via the network 300. The network 300 may beconfigured using a 3G network, a 4G (e.g., LTE) network, or a 5G (e.g.,NR) network. Although the wireless devices 100 a to 100 f maycommunicate with each other through the BSs 200/network 300, thewireless devices 100 a to 100 f may perform direct communication (e.g.,sidelink communication) with each other without passing through theBSs/network. For example, the vehicles 100 b-1 and 100 b-2 may performdirect communication (e.g. Vehicle-to-Vehicle(V2V)/Vehicle-to-everything (V2X) communication). The IoT device (e.g.,a sensor) may perform direct communication with other IoT devices (e.g.,sensors) or other wireless devices 100 a to 100 f.

Wireless communication/connections 150 a, 150 b, or 150 c may beestablished between the wireless devices 100 a to 100 f/BS 200, or BS200/BS 200. Herein, the wireless communication/connections may beestablished through various RATs (e.g., 5G NR) such as uplink/downlinkcommunication 150 a, sidelink communication 150 b (or, D2Dcommunication), or inter BS communication (e.g. relay, Integrated AccessBackhaul (IAB)). The wireless devices and the BSs/the wireless devicesmay transmit/receive radio signals to/from each other through thewireless communication/connections 150 a and 150 b. For example, thewireless communication/connections 150 a and 150 b may transmit/receivesignals through various physical channels. To this end, at least a partof various configuration information configuring processes, varioussignal processing processes (e.g., channel encoding/decoding,modulation/demodulation, and resource mapping/demapping), and resourceallocating processes, for transmitting/receiving radio signals, may beperformed based on the various proposals of the present disclosure.

FIG. 15 shows wireless devices, in accordance with an embodiment of thepresent disclosure.

Referring to FIG. 15, a first wireless device 100 and a second wirelessdevice 200 may transmit radio signals through a variety of RATs (e.g.,LTE and NR). Herein, {the first wireless device 100 and the secondwireless device 200} may correspond to {the wireless device 100 x andthe BS 200} and/or {the wireless device 100 x and the wireless device100 x} of FIG. 14.

The first wireless device 100 may include one or more processors 102 andone or more memories 104 and additionally further include one or moretransceivers 106 and/or one or more antennas 108. The processor(s) 102may control the memory(s) 104 and/or the transceiver(s) 106 and may beconfigured to implement the descriptions, functions, procedures,proposals, methods, and/or operational flowcharts disclosed in thisdocument. For example, the processor(s) 102 may process informationwithin the memory(s) 104 to generate first information/signals and thentransmit radio signals including the first information/signals throughthe transceiver(s) 106. The processor(s) 102 may receive radio signalsincluding second information/signals through the transceiver 106 andthen store information obtained by processing the secondinformation/signals in the memory(s) 104. The memory(s) 104 may beconnected to the processor(s) 102 and may store a variety of informationrelated to operations of the processor(s) 102. For example, thememory(s) 104 may store software code including commands for performinga part or the entirety of processes controlled by the processor(s) 102or for performing the descriptions, functions, procedures, proposals,methods, and/or operational flowcharts disclosed in this document.Herein, the processor(s) 102 and the memory(s) 104 may be a part of acommunication modem/circuit/chip designed to implement RAT (e.g., LTE orNR). The transceiver(s) 106 may be connected to the processor(s) 102 andtransmit and/or receive radio signals through one or more antennas 108.Each of the transceiver(s) 106 may include a transmitter and/or areceiver. The transceiver(s) 106 may be interchangeably used with RadioFrequency (RF) unit(s). In the present disclosure, the wireless devicemay represent a communication modem/circuit/chip.

The second wireless device 200 may include one or more processors 202and one or more memories 204 and additionally further include one ormore transceivers 206 and/or one or more antennas 208. The processor(s)202 may control the memory(s) 204 and/or the transceiver(s) 206 and maybe configured to implement the descriptions, functions, procedures,proposals, methods, and/or operational flowcharts disclosed in thisdocument. For example, the processor(s) 202 may process informationwithin the memory(s) 204 to generate third information/signals and thentransmit radio signals including the third information/signals throughthe transceiver(s) 206. The processor(s) 202 may receive radio signalsincluding fourth information/signals through the transceiver(s) 106 andthen store information obtained by processing the fourthinformation/signals in the memory(s) 204. The memory(s) 204 may beconnected to the processor(s) 202 and may store a variety of informationrelated to operations of the processor(s) 202. For example, thememory(s) 204 may store software code including commands for performinga part or the entirety of processes controlled by the processor(s) 202or for performing the descriptions, functions, procedures, proposals,methods, and/or operational flowcharts disclosed in this document.Herein, the processor(s) 202 and the memory(s) 204 may be a part of acommunication modem/circuit/chip designed to implement RAT (e.g., LTE orNR). The transceiver(s) 206 may be connected to the processor(s) 202 andtransmit and/or receive radio signals through one or more antennas 208.Each of the transceiver(s) 206 may include a transmitter and/or areceiver. The transceiver(s) 206 may be interchangeably used with RFunit(s). In the present disclosure, the wireless device may represent acommunication modem/circuit/chip.

Hereinafter, hardware elements of the wireless devices 100 and 200 willbe described more specifically. One or more protocol layers may beimplemented by, without being limited to, one or more processors 102 and202. For example, the one or more processors 102 and 202 may implementone or more layers (e.g., functional layers such as PHY, MAC, RLC, PDCP,RRC, and SDAP). The one or more processors 102 and 202 may generate oneor more Protocol Data Units (PDUs) and/or one or more Service Data Unit(SDUs) according to the descriptions, functions, procedures, proposals,methods, and/or operational flowcharts disclosed in this document. Theone or more processors 102 and 202 may generate messages, controlinformation, data, or information according to the descriptions,functions, procedures, proposals, methods, and/or operational flowchartsdisclosed in this document. The one or more processors 102 and 202 maygenerate signals (e.g., baseband signals) including PDUs, SDUs,messages, control information, data, or information according to thedescriptions, functions, procedures, proposals, methods, and/oroperational flowcharts disclosed in this document and provide thegenerated signals to the one or more transceivers 106 and 206. The oneor more processors 102 and 202 may receive the signals (e.g., basebandsignals) from the one or more transceivers 106 and 206 and acquire thePDUs, SDUs, messages, control information, data, or informationaccording to the descriptions, functions, procedures, proposals,methods, and/or operational flowcharts disclosed in this document.

The one or more processors 102 and 202 may be referred to ascontrollers, microcontrollers, microprocessors, or microcomputers. Theone or more processors 102 and 202 may be implemented by hardware,firmware, software, or a combination thereof. As an example, one or moreApplication Specific Integrated Circuits (ASICs), one or more DigitalSignal Processors (DSPs), one or more Digital Signal Processing Devices(DSPDs), one or more Programmable Logic Devices (PLDs), or one or moreField Programmable Gate Arrays (FPGAs) may be included in the one ormore processors 102 and 202. The descriptions, functions, procedures,proposals, methods, and/or operational flowcharts disclosed in thisdocument may be implemented using firmware or software and the firmwareor software may be configured to include the modules, procedures, orfunctions. Firmware or software configured to perform the descriptions,functions, procedures, proposals, methods, and/or operational flowchartsdisclosed in this document may be included in the one or more processors102 and 202 or stored in the one or more memories 104 and 204 so as tobe driven by the one or more processors 102 and 202. The descriptions,functions, procedures, proposals, methods, and/or operational flowchartsdisclosed in this document may be implemented using firmware or softwarein the form of code, commands, and/or a set of commands.

The one or more memories 104 and 204 may be connected to the one or moreprocessors 102 and 202 and store various types of data, signals,messages, information, programs, code, instructions, and/or commands.The one or more memories 104 and 204 may be configured by Read-OnlyMemories (ROMs), Random Access Memories (RAMs), Electrically ErasableProgrammable Read-Only Memories (EPROMs), flash memories, hard drives,registers, cash memories, computer-readable storage media, and/orcombinations thereof. The one or more memories 104 and 204 may belocated at the interior and/or exterior of the one or more processors102 and 202. The one or more memories 104 and 204 may be connected tothe one or more processors 102 and 202 through various technologies suchas wired or wireless connection.

The one or more transceivers 106 and 206 may transmit user data, controlinformation, and/or radio signals/channels, mentioned in the methodsand/or operational flowcharts of this document, to one or more otherdevices. The one or more transceivers 106 and 206 may receive user data,control information, and/or radio signals/channels, mentioned in thedescriptions, functions, procedures, proposals, methods, and/oroperational flowcharts disclosed in this document, from one or moreother devices. For example, the one or more transceivers 106 and 206 maybe connected to the one or more processors 102 and 202 and transmit andreceive radio signals. For example, the one or more processors 102 and202 may perform control so that the one or more transceivers 106 and 206may transmit user data, control information, or radio signals to one ormore other devices. The one or more processors 102 and 202 may performcontrol so that the one or more transceivers 106 and 206 may receiveuser data, control information, or radio signals from one or more otherdevices. The one or more transceivers 106 and 206 may be connected tothe one or more antennas 108 and 208 and the one or more transceivers106 and 206 may be configured to transmit and receive user data, controlinformation, and/or radio signals/channels, mentioned in thedescriptions, functions, procedures, proposals, methods, and/oroperational flowcharts disclosed in this document, through the one ormore antennas 108 and 208. In this document, the one or more antennasmay be a plurality of physical antennas or a plurality of logicalantennas (e.g., antenna ports). The one or more transceivers 106 and 206may convert received radio signals/channels etc. from RF band signalsinto baseband signals in order to process received user data, controlinformation, radio signals/channels, etc. using the one or moreprocessors 102 and 202. The one or more transceivers 106 and 206 mayconvert the user data, control information, radio signals/channels, etc.processed using the one or more processors 102 and 202 from the baseband signals into the RF band signals. To this end, the one or moretransceivers 106 and 206 may include (analog) oscillators and/orfilters.

FIG. 16 shows a signal process circuit for a transmission signal, inaccordance with an embodiment of the present disclosure.

Referring to FIG. 16, a signal processing circuit 1000 may includescramblers 1010, modulators 1020, a layer mapper 1030, a precoder 1040,resource mappers 1050, and signal generators 1060. An operation/functionof FIG. 16 may be performed, without being limited to, the processors102 and 202 and/or the transceivers 106 and 206 of FIG. 15. Hardwareelements of FIG. 16 may be implemented by the processors 102 and 202and/or the transceivers 106 and 206 of FIG. 15. For example, blocks 1010to 1060 may be implemented by the processors 102 and 202 of FIG. 15.Alternatively, the blocks 1010 to 1050 may be implemented by theprocessors 102 and 202 of FIG. 15 and the block 1060 may be implementedby the transceivers 106 and 206 of FIG. 15.

Codewords may be converted into radio signals via the signal processingcircuit 1000 of FIG. 16. Herein, the codewords are encoded bit sequencesof information blocks. The information blocks may include transportblocks (e.g., a UL-SCH transport block, a DL-SCH transport block). Theradio signals may be transmitted through various physical channels(e.g., a PUSCH and a PDSCH).

Specifically, the codewords may be converted into scrambled bitsequences by the scramblers 1010. Scramble sequences used for scramblingmay be generated based on an initialization value, and theinitialization value may include ID information of a wireless device.The scrambled bit sequences may be modulated to modulation symbolsequences by the modulators 1020. A modulation scheme may includepi/2-Binary Phase Shift Keying (pi/2-BPSK), m-Phase Shift Keying(m-PSK), and m-Quadrature Amplitude Modulation (m-QAM). Complexmodulation symbol sequences may be mapped to one or more transportlayers by the layer mapper 1030. Modulation symbols of each transportlayer may be mapped (precoded) to corresponding antenna port(s) by theprecoder 1040. Outputs z of the precoder 1040 may be obtained bymultiplying outputs y of the layer mapper 1030 by an N*M precodingmatrix W. Herein, N is the number of antenna ports and M is the numberof transport layers. The precoder 1040 may perform precoding afterperforming transform precoding (e.g., DFT) for complex modulationsymbols. Alternatively, the precoder 1040 may perform precoding withoutperforming transform precoding.

The resource mappers 1050 may map modulation symbols of each antennaport to time-frequency resources. The time-frequency resources mayinclude a plurality of symbols (e.g., a CP-OFDMA symbols and DFT-s-OFDMAsymbols) in the time domain and a plurality of subcarriers in thefrequency domain. The signal generators 1060 may generate radio signalsfrom the mapped modulation symbols and the generated radio signals maybe transmitted to other devices through each antenna. For this purpose,the signal generators 1060 may include Inverse Fast Fourier Transform(IFFT) modules, Cyclic Prefix (CP) inserters, Digital-to-AnalogConverters (DACs), and frequency up-converters.

Signal processing procedures for a signal received in the wirelessdevice may be configured in a reverse manner of the signal processingprocedures 1010 to 1060 of FIG. 16. For example, the wireless devices(e.g., 100 and 200 of FIG. 15) may receive radio signals from theexterior through the antenna ports/transceivers. The received radiosignals may be converted into baseband signals through signal restorers.To this end, the signal restorers may include frequency downlinkconverters, Analog-to-Digital Converters (ADCs), CP remover, and FastFourier Transform (FFT) modules. Next, the baseband signals may berestored to codewords through a resource demapping procedure, apostcoding procedure, a demodulation processor, and a descramblingprocedure. The codewords may be restored to original information blocksthrough decoding. Therefore, a signal processing circuit (notillustrated) for a reception signal may include signal restorers,resource demappers, a postcoder, demodulators, descramblers, anddecoders.

FIG. 17 shows another example of a wireless device, in accordance withan embodiment of the present disclosure. The wireless device may beimplemented in various forms according to a use-case/service (refer toFIG. 14).

Referring to FIG. 17, wireless devices 100 and 200 may correspond to thewireless devices 100 and 200 of FIG. 15 and may be configured by variouselements, components, units/portions, and/or modules. For example, eachof the wireless devices 100 and 200 may include a communication unit110, a control unit 120, a memory unit 130, and additional components140. The communication unit may include a communication circuit 112 andtransceiver(s) 114. For example, the communication circuit 112 mayinclude the one or more processors 102 and 202 and/or the one or morememories 104 and 204 of FIG. 15. For example, the transceiver(s) 114 mayinclude the one or more transceivers 106 and 206 and/or the one or moreantennas 108 and 208 of FIG. 15. The control unit 120 is electricallyconnected to the communication unit 110, the memory 130, and theadditional components 140 and controls overall operation of the wirelessdevices. For example, the control unit 120 may control anelectric/mechanical operation of the wireless device based onprograms/code/commands/information stored in the memory unit 130. Thecontrol unit 120 may transmit the information stored in the memory unit130 to the exterior (e.g., other communication devices) via thecommunication unit 110 through a wireless/wired interface or store, inthe memory unit 130, information received through the wireless/wiredinterface from the exterior (e.g., other communication devices) via thecommunication unit 110.

The additional components 140 may be variously configured according totypes of wireless devices. For example, the additional components 140may include at least one of a power unit/battery, input/output (I/O)unit, a driving unit, and a computing unit. The wireless device may beimplemented in the form of, without being limited to, the robot (100 aof FIG. 14), the vehicles (100 b-1 and 100 b-2 of FIG. 14), the XRdevice (100 c of FIG. 14), the hand-held device (100 d of FIG. 14), thehome appliance (100 e of FIG. 14), the IoT device (100 f of FIG. 14), adigital broadcast terminal, a hologram device, a public safety device,an MTC device, a medicine device, a fintech device (or a financedevice), a security device, a climate/environment device, the AIserver/device (400 of FIG. 14), the BSs (200 of FIG. 14), a networknode, etc. The wireless device may be used in a mobile or fixed placeaccording to a use-example/service.

In FIG. 17, the entirety of the various elements, components,units/portions, and/or modules in the wireless devices 100 and 200 maybe connected to each other through a wired interface or at least a partthereof may be wirelessly connected through the communication unit 110.For example, in each of the wireless devices 100 and 200, the controlunit 120 and the communication unit 110 may be connected by wire and thecontrol unit 120 and first units (e.g., 130 and 140) may be wirelesslyconnected through the communication unit 110. Each element, component,unit/portion, and/or module within the wireless devices 100 and 200 mayfurther include one or more elements. For example, the control unit 120may be configured by a set of one or more processors. As an example, thecontrol unit 120 may be configured by a set of a communication controlprocessor, an application processor, an Electronic Control Unit (ECU), agraphical processing unit, and a memory control processor. As anotherexample, the memory 130 may be configured by a Random Access Memory(RAM), a Dynamic RAM (DRAM), a Read Only Memory (ROM)), a flash memory,a volatile memory, a non-volatile memory, and/or a combination thereof.

Hereinafter, an example of implementing FIG. 17 will be described indetail with reference to the drawings.

FIG. 18 shows a hand-held device, in accordance with an embodiment ofthe present disclosure. The hand-held device may include a smartphone, asmartpad, a wearable device (e.g., a smartwatch or a smartglasses), or aportable computer (e.g., a notebook). The hand-held device may bereferred to as a mobile station (MS), a user terminal (UT), a MobileSubscriber Station (MSS), a Subscriber Station (SS), an Advanced MobileStation (AMS), or a Wireless Terminal (WT).

Referring to FIG. 18, a hand-held device 100 may include an antenna unit108, a communication unit 110, a control unit 120, a memory unit 130, apower supply unit 140 a, an interface unit 140 b, and an I/O unit 140 c.The antenna unit 108 may be configured as a part of the communicationunit 110. Blocks 110 to 130/140 a to 140 c correspond to the blocks 110to 130/140 of FIG. 17, respectively.

The communication unit 110 may transmit and receive signals (e.g., dataand control signals) to and from other wireless devices or BSs. Thecontrol unit 120 may perform various operations by controllingconstituent elements of the hand-held device 100. The control unit 120may include an Application Processor (AP). The memory unit 130 may storedata/parameters/programs/code/commands needed to drive the hand-helddevice 100. The memory unit 130 may store input/output data/information.The power supply unit 140 a may supply power to the hand-held device 100and include a wired/wireless charging circuit, a battery, etc. Theinterface unit 140 b may support connection of the hand-held device 100to other external devices. The interface unit 140 b may include variousports (e.g., an audio I/O port and a video I/O port) for connection withexternal devices. The I/O unit 140 c may input or output videoinformation/signals, audio information/signals, data, and/or informationinput by a user. The I/O unit 140 c may include a camera, a microphone,a user input unit, a display unit 140 d, a speaker, and/or a hapticmodule.

As an example, in the case of data communication, the I/O unit 140 c mayacquire information/signals (e.g., touch, text, voice, images, or video)input by a user and the acquired information/signals may be stored inthe memory unit 130. The communication unit 110 may convert theinformation/signals stored in the memory into radio signals and transmitthe converted radio signals to other wireless devices directly or to aBS. The communication unit 110 may receive radio signals from otherwireless devices or the BS and then restore the received radio signalsinto original information/signals. The restored information/signals maybe stored in the memory unit 130 and may be output as various types(e.g., text, voice, images, video, or haptic) through the I/O unit 140c.

FIG. 19 shows a vehicle or an autonomous vehicle, in accordance with anembodiment of the present disclosure. The vehicle or autonomous vehiclemay be implemented by a mobile robot, a car, a train, a manned/unmannedAerial Vehicle (AV), a ship, etc.

Referring to FIG. 19, a vehicle or autonomous vehicle 100 may include anantenna unit 108, a communication unit 110, a control unit 120, adriving unit 140 a, a power supply unit 140 b, a sensor unit 140 c, andan autonomous driving unit 140 d. The antenna unit 108 may be configuredas a part of the communication unit 110. The blocks 110/130/140 a to 140d correspond to the blocks 110/130/140 of FIG. 17, respectively.

The communication unit 110 may transmit and receive signals (e.g., dataand control signals) to and from external devices such as othervehicles, BSs (e.g., gNBs and road side units), and servers. The controlunit 120 may perform various operations by controlling elements of thevehicle or the autonomous vehicle 100. The control unit 120 may includean Electronic Control Unit (ECU). The driving unit 140 a may cause thevehicle or the autonomous vehicle 100 to drive on a road. The drivingunit 140 a may include an engine, a motor, a powertrain, a wheel, abrake, a steering device, etc. The power supply unit 140 b may supplypower to the vehicle or the autonomous vehicle 100 and include awired/wireless charging circuit, a battery, etc. The sensor unit 140 cmay acquire a vehicle state, ambient environment information, userinformation, etc. The sensor unit 140 c may include an InertialMeasurement Unit (IMU) sensor, a collision sensor, a wheel sensor, aspeed sensor, a slope sensor, a weight sensor, a heading sensor, aposition module, a vehicle forward/backward sensor, a battery sensor, afuel sensor, a tire sensor, a steering sensor, a temperature sensor, ahumidity sensor, an ultrasonic sensor, an illumination sensor, a pedalposition sensor, etc. The autonomous driving unit 140 d may implementtechnology for maintaining a lane on which a vehicle is driving,technology for automatically adjusting speed, such as adaptive cruisecontrol, technology for autonomously driving along a determined path,technology for driving by automatically setting a path if a destinationis set, and the like.

For example, the communication unit 110 may receive map data, trafficinformation data, etc. from an external server. The autonomous drivingunit 140 d may generate an autonomous driving path and a driving planfrom the obtained data. The control unit 120 may control the drivingunit 140 a such that the vehicle or the autonomous vehicle 100 may movealong the autonomous driving path according to the driving plan (e.g.,speed/direction control). In the middle of autonomous driving, thecommunication unit 110 may aperiodically/periodically acquire recenttraffic information data from the external server and acquiresurrounding traffic information data from neighboring vehicles. In themiddle of autonomous driving, the sensor unit 140 c may obtain a vehiclestate and/or surrounding environment information. The autonomous drivingunit 140 d may update the autonomous driving path and the driving planbased on the newly obtained data/information. The communication unit 110may transfer information about a vehicle position, the autonomousdriving path, and/or the driving plan to the external server. Theexternal server may predict traffic information data using AItechnology, etc., based on the information collected from vehicles orautonomous vehicles and provide the predicted traffic information datato the vehicles or the autonomous vehicles.

Claims in the present description can be combined in a various way. Forinstance, technical features in method claims of the present descriptioncan be combined to be implemented or performed in an apparatus, andtechnical features in apparatus claims can be combined to be implementedor performed in a method. Further, technical features in method claim(s)and apparatus claim(s) can be combined to be implemented or performed inan apparatus. Further, technical features in method claim(s) andapparatus claim(s) can be combined to be implemented or performed in amethod.

What is claimed is:
 1. A method for performing wireless communication bya first device, the method comprising: establishing a PC5 radio resourcecontrol (RRC) connection with a second device, wherein the second deviceis a relay device for relaying communication between the first deviceand a base station; transmitting, to the second device, a first sidelinkcontrol information (SCI) through a physical sidelink control channel(PSCCH); transmitting, to the second device, a second SCI and datathrough a physical sidelink shared channel (PSSCH) related to the PSCCH;determining a physical sidelink feedback channel (PSFCH) resource basedon an index of a subchannel and a slot related to the PSSCH; based onfailure to receive a sidelink (SL) hybrid automatic repeat request(HARQ) feedback for the data from the second device based on the PSFCHresource, detecting a radio link failure (RLF) for the PC5 RRCconnection between the first device and the second device; andperforming reselection for the relay device based on detecting the RLFfor the PC5 RRC connection.
 2. The method of claim 1, wherein, based ona number of failure to receive the SL HARQ feedback reaching a firstthreshold, the RLF for the PC5 RRC connection is detected by the firstdevice.
 3. The method of claim 1, wherein, based on a number ofout-of-syncs (OOSs) transferred from a physical layer of the firstdevice to an RRC layer of the first device reaching a second threshold,the RLF for the PC5 RRC connection is detected by the first device. 4.The method of claim 1, wherein, based on detecting an integrity checkfailure, the RLF for the PC5 RRC connection is detected by the firstdevice.
 5. The method of claim 1, wherein the first device is allowed toperform reselection for the relay device while a timer is running. 6.The method of claim 5, wherein the timer is configured for the firstdevice.
 7. The method of claim 1, wherein, based on detecting the RLFfor the PC5 RRC connection, a signaling radio bearer (SRB) and a dataradio bearer (DRB) related to the PC5 RRC connection between the firstdevice and the second device is suspended.
 8. The method of claim 1,wherein, based on detecting the RLF for the PC5 RRC connection,reselection for the relay device is triggered by the first device. 9.The method of claim 1, further comprising: based on detecting the RLFfor the PC5 RRC connection, transmitting a message including (i) a causeof the RLF, (ii) a list of candidate relay devices, and (iii) a resultof channel measurement for the candidate relay devices, to the basestation.
 10. The method of claim 9, wherein the cause of the RLFincludes at least one of information representing that the RLF isrelated to expiration of a timer, information representing that the RLFis related to a packet data convergence protocol (PDCP) layer,information representing that the RLF is related to a radio link control(RLC) layer, information representing that the RLF is related to amedium access control (MAC) layer, or information representing that theRLF is related to a physical layer.
 11. The method of claim 1, wherein,the reselection for the relay device comprises: performing channelmeasurement for at least one candidate relay device; and selecting athird device for which a result of the channel measurement exceeds athird threshold from among the at least one candidate relay device asthe relay device.
 12. The method of claim 11, further comprising:transmitting a message including UE ID of the third device to the basestation.
 13. The method of claim 11, further comprising: establishingthe PC5 RRC connection with the third device.
 14. The method of claim11, wherein the third threshold is configured by the base station to thefirst device.
 15. The method of claim 1, further comprising: based onfailure of the reselection for the relay device, transmitting a messageincluding (i) information representing the failure of the reselectionand (ii) a cause of the RLF, to the base station.
 16. The method ofclaim 15, wherein the cause of the RLF includes at least one ofinformation representing that the RLF is related to expiration of atimer, information representing that the RLF is related to a packet dataconvergence protocol (PDCP) layer, information representing that the RLFis related to a radio link control (RLC) layer, information representingthat the RLF is related to a medium access control (MAC) layer, orinformation representing that the RLF is related to a physical layer.17. A first device configured to perform wireless communication, thefirst device comprising: one or more memories storing instructions; oneor more transceivers; and one or more processors connected to the one ormore memories and the one or more transceivers, wherein the one or moreprocessors execute the instructions to: establish a PC5 radio resourcecontrol (RRC) connection with a second device, wherein the second deviceis a relay device for relaying communication between the first deviceand a base station; transmit, to the second device, a first sidelinkcontrol information (SCI) through a physical sidelink control channel(PSCCH); transmit, to the second device, a second SCI and data through aphysical sidelink shared channel (PSSCH) related to the PSCCH; determinea physical sidelink feedback channel (PSFCH) resource based on an indexof a subchannel and a slot related to the PSSCH; based on failure toreceive a sidelink (SL) hybrid automatic repeat request (HARQ) feedbackfor the data from the second device based on the PSFCH resource, detecta radio link failure (RLF) for the PC5 RRC connection between the firstdevice and the second device; and perform reselection for the relaydevice based on detecting the RLF for the PC5 RRC connection.
 18. Thefirst device of claim 17, wherein, based on a number of failure toreceive the SL HARQ feedback reaching a first threshold, the RLF for thePC5 RRC connection is detected by the first device.
 19. An apparatusconfigured to control a first user equipment (UE), the apparatuscomprising: one or more processors; and one or more memories operablyconnected to the one or more processors and storing instructions,wherein the one or more processors execute the instructions to:establish a PC5 radio resource control (RRC) connection with a secondUE, wherein the second UE is a relay UE for relaying communicationbetween the first UE and a base station; transmit, to the second UE, afirst sidelink control information (SCI) through a physical sidelinkcontrol channel (PSCCH); transmit, to the second UE, a second SCI anddata through a physical sidelink shared channel (PSSCH) related to thePSCCH; determine a physical sidelink feedback channel (PSFCH) resourcebased on an index of a subchannel and a slot related to the PSSCH; basedon failure to receive a sidelink (SL) hybrid automatic repeat request(HARQ) feedback for the data from the second UE based on the PSFCHresource, detect a radio link failure (RLF) for the PC5 RRC connectionbetween the first UE and the second UE; and perform reselection for therelay UE based on detecting the RLF for the PC5 RRC connection.
 20. Theapparatus of claim 19, wherein, based on a number of failure to receivethe SL HARQ feedback reaching a first threshold, the RLF for the PC5 RRCconnection is detected by the first UE.